Rapidly oscillating magnetic fields proposed for spectroscopic studies and in use for snapshot NMR imaging have sufficient amplitude to cause neuromuscular stimulation and possibly cardiac excitation in humans under pathological conditions. This project will determine the thresholds for cardiac excitation by oscillating magnetic fields as a function of amplitude, orientation and frequency in intact and open-chest dog models under normal and ischemic conditions. The measurements and finite element modeling will be used to predict excitation and fibrillation probabilities in man under a variety of NMR imaging and spectroscopy protocols involving rapidly switched gradients. Specific aims include measurements of the distribution of electrical properties in the canine heart, modeling of field distributions responsible for the excitation by finite element analysis and in vitro measurements to determine the electrophysiology and stimulation mechanisms of oscillating magnetic fields relative to the electrophysiology of oscillating electric fields from direct electrode potential changes. Studies involve collaboration with Boston University for dog finite element analyses, Duke University for electrophysiology studies and University of Pennsylvania for tissue anisotropy studies. The final objective is to ensure hazard-free application of NMR in human cardiovascular studies.